For the birds: April Fool's Day bird - whose egg are you?

Male and female Brown-headed Cowbirds
Photo by Dasha Gudalewicz

By Christine Southwick

Scenario: within a warbler’s nest is a larger egg than all the others. It will most likely hatch before the native eggs, develop faster, and demand more food than the host mother / parents can likely supply to the whole brood.

Who claims responsibility for this brood parasitic behavior?

In our area, it is the Brown-headed Cowbird. It’s not really the cowbird’s fault. This species was a plains bird that followed the bison, eating the bugs and seeds disturbed by those large herbivores. To survive, the Brown-headed Cowbird evolved to laying eggs in other species' nests since they were not going to be around to brood / raise the eggs they laid. Good for the cowbirds, bad for the 144 (plus) species that fall prey to raising a bigger, hungrier, cowbird off-spring.

A female Brown-headed Cowbird watches birds making nests, waits until there is an egg in the nest, and then lays one egg. If there are several eggs already in the nest, she may remove one, often eating it to replenish her calcium so that she can continue to lay her up to 40 eggs per season that others will raise.

Brown-headed Cowbird being fed by Lazui Bunting
Photo by Dasha Gudalewicz

Small birds like warblers and kinglets are the most in danger. The eggs are too big for them to remove. Some, like the Yellow Warbler, recognize cowbird eggs, build a new nest over the top of their first nest, and hope the cowbird doesn’t return. Some larger species like jays, robins, and orioles, recognize cowbird eggs and eject them from their nests.

Brown-headed Cowbird being fed by Song Sparrow
Photo by John Riegsecker

But most, like Song Sparrows, Chipping Sparrows, and juncos, raise them as one of their own. Cowbird nestlings, being bigger and more demanding, tax the host’s resources to the point that the host’s nestlings often starve. Endangered species can ill afford to lose a brood due to cowbird parasitism.

Cowbirds are one of the most important causes of songbird declines in North America.

Brown-headed Cowbird being fed by Junco
Photo by Kelly McAllister

Brown-headed Cowbirds are a relatively recent occurrence in Western Washington. Since they avoid forests, preferring feed-lots and brushy fields, they were not seen regularly here until 1955, after enough forests were cleared to create large open areas.

Song Sparrows and Yellow Warblers, two of our local birds, are the two most parasitized species in the US.

(Fun note: American Goldfinches, being totally vegetarian, don’t have to worry since cowbird nestlings starve without protein.)